Sleep is a naturally recurring state characterized by reduced or absent consciousness, relatively suspended sensory activity, and inactivity of nearly all voluntary muscles. Sleep architecture refers to the basic structural organization of normal sleep.
There are two distinct states that alternate in 90 minute cycles and reflect differing levels of brain activity. Each sleep cycle consists of non-rapid eye movement (NREM) and rapid eye movement (REM) activities, both states repeat over and over again during a night's sleep. NREM sleep is further subdivided into four stages. Each state is characterized by a different type of brain wave.
Stage N1 is of light sleep, which is considered a transition between wakefulness and sleep and usually accounts for 5-10% of total sleep time. This stage is characterized by alpha brain waves having a frequency 8-13 Hz. An individual can be easily awakened during this period.
Stage N2 occurs throughout the sleep period and represents 40-50% of the total sleep time. This stage is characterized by theta brain waves ranging from 4 to 8 Hz. During stage N2, brain waves slow down with occasional bursts of rapid waves.
Stages III and IV are distinguished from each other only by the percentage of delta wave activity with a frequency oscillation between 0 and 4 Hz. Together these two stages represent up to 20% of total sleep time. Stages N3 and N4 represent deep sleep, during which all eye and muscle movement ceases. It is difficult to wake up an individual during these 2 stages; these have been combined by the American Academy of Sleep Medicine as stage N3 and are called slow wave or delta sleep. Slow wave sleep provides the most recuperative effect and defines the quality of sleep.